1. Field of the Invention
The invention relates to an apparatus and a method which determine whether an oxygen sensor has deteriorated, and which are applied to an engine in which a feedback control of an air-fuel ratio is performed by calculating an air-fuel ratio correction coefficient based on an output of the oxygen sensor.
2. Description of the Related Art
As is generally known, an oxygen sensor that detects an oxygen concentration of exhaust gas is provided in an exhaust system, and a feedback control of an air-fuel ratio is performed based on an output of the oxygen sensor in many types of engines. In such engines, when it is detected that the air-fuel ratio is richer than a target air-fuel ratio based on the output of the oxygen sensor, a decrease correction of a fuel injection amount is performed. When it is detected that the air-fuel ratio is leaner than the air-fuel ratio based on the output of the oxygen sensor, an increase correction of the fuel injection amount is performed. Thus, the air fuel ratio is controlled to the target air-fuel ratio. The correction of the fuel injection amount is generally performed using a skip correction and an integral correction. In the skip correction, the air-fuel ratio correction coefficient is increased or decreased by a predetermined amount at one time when it is detected, based on the output of the oxygen sensor, that the air-fuel ratio changes from lean to rich or rich to lean. In the integral correction, the air-fuel ratio correction coefficient is gradually increased or decreased during a period in which the output of the oxygen sensor shows that the air-fuel ratio is lean or rich.
The characteristics of the oxygen sensor used for such a feedback control of the air-fuel ratio, such as internal resistance, electromotive force, and response time, may change due to thermal deterioration or the like, and the accuracy of the air-fuel ratio control may decrease due to the change in the characteristics. Therefore, in most of the engines in which the aforementioned feedback control of the air-fuel ratio is performed, a determination as to whether the oxygen sensor has deteriorated is made in order to prevent such deterioration of the accuracy.
Conventionally, the determination concerning deterioration of the oxygen sensor is made by determining that response has deteriorated when an inversion cycle of the output of the oxygen sensor is equal to or longer than a deterioration determining value, as disclosed by Japanese Patent Laid-Open Publication No. 6-50200. The inversion cycle of the output of the oxygen sensor indicates both a time period from when the output of the oxygen sensor becomes lean until when the output becomes rich, and a time period from when the output becomes rich until when the output becomes lean.
Japanese Patent Laid-Open Publication No. 11-166438 proposes a deterioration determining apparatus for an oxygen sensor in which a deterioration determining value is set so as to be variable according to an integrated value of an intake air amount when a determination concerning deterioration of the oxygen sensor is made based on an inversion cycle of the oxygen sensor. In the deterioration determining apparatus, the determination concerning deterioration of the oxygen sensor can be appropriately made, irrespective of a change in the inversion cycle of the oxygen sensor due to a change in a flow speed of intake air and exhaust gas according to the intake air amount.
In the feedback control of the air-fuel ratio, the response of the aforementioned air-fuel ratio correction coefficient with respect to the lean-rich inversion of the output of the oxygen sensor may be set so as to be variable according to a situation. The response of the air-fuel ratio correction coefficient is set so as to be variable by changing an integral correction amount or a skip correction amount. The integral correction amount is an increase/decrease rate of the air-fuel ratio correction coefficient at the time of the aforementioned integral correction. The skip correction amount is an increase/decrease amount of the air-fuel ratio correction coefficient at the time of the aforementioned skip correction. For example, when the integral correction amount or the skip correction amount is decreased, a degree of change in the air-fuel ratio correction coefficient in response to the lean-rich inversion of the output of the oxygen sensor decreases, and the response of the air-fuel ratio correction coefficient decreases.
Also, the feedback center of the air-fuel ratio correction coefficient may be adjusted by setting the integral correction amounts of the air-fuel ratio correction coefficient at the time of rich air-fuel ratio and at the time of lean air-fuel ratio to different values, or setting the skip correction amounts at the time of rich air-fuel ratio and at the time of lean air-fuel ratio to different values so that the response of the air-fuel ratio correction coefficient at the time of rich air-fuel ratio becomes different from the response at the time of lean air-fuel ratio. For example, when the response of the air-fuel ratio correction coefficient at the time of rich air-fuel ratio is made lower than the response at the time of lean air-fuel ratio by decreasing the skip correction amount when the output of the oxygen sensor is inverted from lean to rich, or the integral correction amount when the output of the oxygen sensor is rich, the feedback center of the air-fuel ratio correction coefficient is deviated to the rich side with respect to a value corresponding to the target air-fuel ratio.
Thus, when the response of the air-fuel ratio correction coefficient is changed, the inversion cycle of the output of the oxygen sensor is changed, irrespective of the response of the oxygen sensor itself. For example, when the integral correction amount or the skip correction amount is decreased so that the response of the air-fuel ratio correction coefficient is decreased, the inversion cycle of the output of the oxygen sensor is increased even if the response of the oxygen sensor itself is good.
The inversion cycle of the output of the oxygen sensor is changed according to the set response of the air-fuel ratio correction coefficient, in addition to deterioration of the oxygen sensor and the intake air amount. However, in the aforementioned conventional determination mode, it is not possible to appropriately cope with the change in the inversion cycle due to the response of the air-fuel ratio correction coefficient, and therefore a wrong determination may be made.